They follow a two-level hierarchy of topic categories. The implementation of navigating the topic hierarchy works pretty well, with a nice breadcrumb trail, and a clearly delineated page layout. Additionally there's a "tag" system offering a second index method.
At the bottom of the page is a Submit Podcast link, and anybody can tell them about a podcast.
An interesting thing is if you are registered with Plugged, and registered with Twitter, and logged into Plugged, then Plugged will tell Twitter what you're listening to. That way you can natteringly annoy your twittees about what you're listening to on Plugged. Supposedly this is cool, but maybe since I'm older than 30 this doesn't make sense, or maybe it's because the Twitter site is so slow that I gave up trying to use it.
A registered user can remember favorite shows, can write reviews of shows, can recommend shows, can share recommendations, etc.